1. Field of the Invention
The present invention pertains to medical equipment, and more particularly, to a therapeutic ultrasound system for ablating obstructions within tubular anatomical structures such as blood vessels.
2. Description of the Prior Art
A number of ultrasound systems and devices have heretofore been proposed for use in ablating or removing obstructive material from blood vessels. However, all of these systems and devices generally encounter three types of problems which are not always adequately addressed by these systems and devices.
One type of problem relates generally to the effective transmission of ultrasound energy from an ultrasound source to the distal tip of the device where the ultrasound energy is applied to ablate or remove obstructive material. Since the ultrasound source, such as a transducer, is usually located outside the human body, it is necessary to deliver the ultrasound energy over a long distance, such as about 150 cm, along an ultrasound transmission wire from the source to the distal tip. Attenuation of the acoustical energy along the length of the transmission wire means that the energy reaching the distal tip is reduced. To ensure that sufficient energy reaches the distal tip, a greater amount of energy must be delivered along the transmission wire from the source to the distal tip. This transmission of increased energy along the transmission wire may increase the fatigue experienced by the transmission wire at certain critical locations, such as at the connection between the transducer and the transmission wire. This fatigue and any associated stress may cause the transmission wire to break.
In this regard, the size of the proximal end of the transmission wire cannot be large. The proximal end of the transmission wire is usually bent while moving the ultrasound catheter back and forth during interventional procedures. A larger proximal end for a transmission wire will cause higher attenuation than a smaller proximal end, and provides a larger mass to expand and contract during the delivery of ultrasound energy.
Another type of problem relates to the heat that is built up from the transmission of ultrasound energy along the transmission wire. Many ultrasound transmission wires are made of superelastic alloys which exhibit elasticity within a specific temperature range, usually between 10 degrees Celsius and 50 degrees Celsius. However, during the delivery of ultrasound energy, the temperature of the transmission wire may reach 100 to 200 degrees Celsius, at which the transmission wire may lose its superelasticity and may experience mechanical deformations at portions that are bent when exposed to the high temperatures. The high temperatures may also cause the propagated energy to be lost more rapidly and transferred to heat, thereby reducing the efficacy of the ultrasound transmission wire.
Conventional ultrasound systems typically infuse a coolant fluid (usually 0.9% NaCl solution) through the irrigation lumen of an ultrasound catheter to bathe the transmission wire. To maintain the transmission wire within the desired temperature range of 10-50 degrees Celsius, the irrigation rate of the coolant fluid needs to be dramatically increased. However, there are two limitations to this approach. First, endovascular catheters usually have small inner and outer diameters that range between 0.5 to 3 mm. Therefore, the volume of fluid that can be delivered through the catheter is relatively small. Second, there is a limit to the amount of irrigant that can be delivered and left in the body of the patient during any interventional procedure, and this amount of irrigant should not exceed 500-1,000 cm3. In addition to these two limitations, increased irrigation fluid pressure may cause local tissue damage.
Thus, there still exists a need in the art for improved ultrasound systems having ultrasound devices or catheters which address the aforementioned problems.